1. Field of the Invention
The present invention relates to surface acoustic wave devices used for mobile communication equipment and their manufacturing method.
2. Related Art of the Invention
Due to the development of mobile communication, there is a growing demand for the improvement of the performance of surface acoustic wave devices that are one of the key devices in manufacturing equipment. The characteristics of surface acoustic wave devices depend on the electromechanical coupling coefficient, delay time temperature coefficient, and surface acoustic wave propagation velocity of piezoelectric substrates. Common piezoelectric substrates are now formed of piezoelectric monocrystal such as crystal, lithium tantalate, or lithium niobate. Due to the anisotropy of the piezoelectric monocrystal, substrates of the same material may have different characteristics depending on their cut angle or propagating direction. Thus, such substrates are selected depending on their applications. In general, the temperature coefficient of frequency (TCF) of these piezoelectric substrates increases with increasing electromechanical coupling coefficient, while it decreases with decreasing TCF, and substrate materials have been required that have a large electromechanical coupling coefficient and a small TCF.
In addition, various mobile communication systems are used and the working frequency band has spread from a conventional 800-MHz band to a 1.9-GHz band. The PCS system in the U.S. and the PCN system in Europe are mobile communication systems that use the 1.9-GHz band and that have a very small difference in frequency (20 MHz) between the transmission and reception bands. Thus, if, for example, a transmission filter is used, it is very difficult to achieve sufficient attenuation in the reception band. When a surface acoustic wave filter is used for these systems, piezoelectric substrates formed of lithium tantalate or niobate having a large electromechanical coupling coefficient are normally used in order to provide a pass band. Due to the large TCF of such piezoelectric substrates (for example, about -35 ppm/.degree. C. in lithium tantalate for 36.degree. Y-cut and X propagation), however, only 10-odd MHz of interval can be substantially provided between the transmission and reception bands taking the operating temperature range and manufacturing deviances into consideration. Consequently, in the above example of a transmission filter, it is further difficult to achieve sufficient attenuation in the reception band. These factors further enhance a demand for piezoelectric substrates having a large electromechanical coupling coefficient and excellent temperature characteristics.
Various approaches have been executed to improve the TCF of surface acoustic wave devices. For example, (1) the well known methods disclosed in J. Appl. Phys. (Vol. 50, pp. 1360-1369, 1979) and IEEE Transactions Sonics and Ultrasonics (Vol. SU-31, pp. 51-57, 1984) improve the TCF of surface acoustic wave devices by forming on lithium tantalate or niobate a silicon oxide film (SiO.sub.2) of a TCF with an opposite sign. (2) In addition, the method disclosed in IEEE Transactions Ultrasonics, Ferroelectrics, and Frequency Control (Vol. 41, pp. 872-875, 1994) forms a polarization inverting layer on the surface of a piezoelectric substrate to allow the electrostatic short-circuit effect of the piezoelectric to be used to control the TCF of a surface acoustic wave device. (3) A method has also been proposed that directly joins different piezoelectric substrates together to provide a piezoelectric substrate having new piezoelectric characteristics.
Conventional surface acoustic wave devices are described below.
First, a conventional surface acoustic wave device is described in which a silicon oxide film is formed on an existing piezoelectric substrate. FIG. 9 is a sectional view of a conventional surface acoustic wave device in which a silicon oxide film is formed on a piezoelectric substrate. In this figure, 201 is a piezoelectric substrate, 203 is a comb-like electrode, and 204 is a silicon oxide film. The piezoelectric substrate 201 comprises lithium tantalate or niobate. This surface acoustic wave device is fabricated by forming the comb-like electrode 203 on the piezoelectric substrate 201, and using a sputtering method to form the silicon oxide film 204 on the piezoelectric substrate 201 on which the comb-like electrode 203 is formed. The piezoelectric characteristics vary depending on the thickness of silicon oxide, and a zero temperature coefficient is obtained at a certain thickness (normally expressed by normalizing the surface acoustic wavelength).
Next, a conventional surface acoustic wave device is discussed in which a polarization inverting layer is formed on the surface of a piezoelectric substrate. FIG. 10 is a sectional view of a conventional surface acoustic wave device in which a polarization inverting layer is formed on the surface of a piezoelectric substrate. In this figure, 201 is the piezoelectric substrate, 203 is the comb-like electrode, and 205 is a polarization inverting layer. This surface acoustic wave device is fabricated by forming the polarization inverting layer 205 on the front surface of the piezoelectric substrate 201 and then forming the comb-like electrode 203. When the polarization inverting layer 205 has a certain depth, the electrostatic short-circuit effect of this layer 205 improves its temperature characteristics compared to existing piezoelectric substrates.
In addition, FIG. 11 is a sectional view showing a configuration of a conventional surface acoustic wave device wherein piezoelectric monocrystals are directly joined together to provide new piezoelectric characteristics. In this figure, 201 is a main substrate consisting of a first piezoelectric substrate, 202 is a supplementary substrate consisting of a second piezoelectric substrate, and 203 is a comb-like electrode. According to this configuration, a surface acoustic wave device with new characteristics is provided by reducing the thickness of the main substrate 201 below one surface acoustic wavelength to excite a surface acoustic wave in a mode different from that of a surface acoustic wave that propagates along the main substrate.
These conventional surface acoustic wave devices, however, have the following problems.
First, while the silicon oxide film or polarization inverting layer can improve the temperature characteristics, the characteristics of the piezoelectric substrate inevitably change. That is, the surface acoustic wave velocity may vary, the propagation loss of surface acoustic waves may increase, the electromechanical coupling coefficient may vary, or unwanted spurious responses may occur. Furthermore, if the silicon oxide film is used, the varying thickness of this film may cause the piezoelectric characteristics and surface acoustic wave velocity of the piezoelectric substrate to vary, thereby hindering manufacturing deviances from being controlled. The film quality of silicon oxide may cause the characteristics of the substrate to change. Similarly, if the polarization inverting layer is used, varying the depth of the polarization inverting layer may cause the piezoelectric characteristics and surface acoustic wave velocity of the piezoelectric substrate to change.
On the other hand, the conventional surface acoustic wave device using a direct junction requires the thickness of the first piezoelectric substrate, that is, the main substrate to be reduced accurately, thereby preventing high frequencies from being used for the process.